Visual C++ distinguishes console application (which open as a command prompt) and windows application (which create their own window).
The first one assumes a "main()" function while the latter one assumes a "WinMain()" function (which is not available right now and that's what the linker is trying to tell you).
So all you need to do is to configure your project to be a console application.
Go to the project properties page (this one
In the tree on the left open "linker" and "system".
On the right set the value "subsystem" to "console".